1. Field of the Invention
This invention relates generally to the field of compression release retarders for internal combustion engines. More particularly, it relates to a compression release engine retarder employing an hydraulic valve actuating mechanism wherein during the retarding mode of operation, the engine is converted from the normal four-stroke cycle to a two-stroke cycle thereby doubling the number of compression release events per unit of time.
2. Prior Art
Engine retarders of the compression release type are well-known in the art. Such engine retarders are designed to convert, temporarily, an internal combustion engine of the spark ignition or compression ignition type into an air compressor so as to develop a retarding horsepower which may be a substantial portion of the operating horsepower developed by the engine.
The compression release engine retarder of the type disclosed in Cummins U.S. Pat. No. 3,220,392 employs an hydraulic system wherein the motion of a master piston controls the motion of a slave piston which, in turn, opens the exhaust valve of the internal combustion engine near the end of the compression stroke whereby the work done in compressing the intake air is not recovered during the expansion or "power" stroke, but, instead, is dissipated through the exhaust and radiator system of the vehicle. The master piston is customarily driven by a pushtube controlled by a cam on the engine camshaft which may be associated with the fuel injector of the cylinder involved or with the intake or exhaust valve of another cylinder.
Other mechanisms may also be used to produce the compression release effect. In Jonsson U.S. Pat. No. 3,367,312, the exhaust valves are sequentially opened near the end of the compression stroke by a separate cam profile formed on the exhaust valve cam and actuated by oscillating the axis of the rocker arm shaft or providing a lost motion mechanism in the rocker arm. See also Cartledge U.S. Pat. No. 3,809,033 which discloses a compression release retarder employing a dual-action cam and a rocker arm having an hydraulically extensible lash take-up piston.
In Pelizzoni U.S. Pat. No. 3,786,792 a system for varying the valve timing for a multi-cylinder engine is disclosed in order to improve, inter alia, the compression release retarding effect. The mechanism disclosed includes hydraulic means to lengthen the valve train so as to utilize a secondary cam profile. The valve train may be lengthened, for example, by increasing the length of the pushtube or providing an extension from the rocker arm.
In Dreisin U.S. Pat. No. 3,859,970 an additional cam is provided on the camshaft to operate a pump which, in turn, operates an hydraulic lifter to move the desired exhaust or intake valve pushtube.
Another approach to compression release retarding involves holding either the exhaust or intake valves, or both, partially open during the retarding operation. A mechanism designed to accomplish this result is disclosed in the Siegler U.S. Pat. No. 3,547,087.
Despite the various mechanisms disclosed in the prior art, this art all relates to the standard four-stroke cycle engine which provides one compression stroke per cylinder and therefore one compression release event per cylinder for every two revolutions of the crankshaft.
Since the issuance of the basic compression release patents, including the Cummins U.S. Pat. No. 3,220,392, development efforts have been directed toward improving the retarding horsepower by improving the timing of the compression release event (Custer U.S. Pat. No. 4,398,510), preventing overtravel of the slave piston (Laas U.S. Pat. No. 3,405,699), preventing overpressure of the hydraulic system (Egan U.S. Pat. No. 4,150,640), preventing overload of the injector pushtube or camshaft (Sickler U.S. Pat. No. 4,271,796) and increasing the inlet manifold pressure during retarding (Price U.S. Pat. No. 4,296,605). However, in each instance the engine continues to operate in the standard four-stroke cycle mode so as to produce one compression release event per cylinder for every two crankshaft revolutions.